Crude oil is the world's main source of hydrocarbons used as fuel and petrochemical feedstock. At the same time, petroleum and petroleum-based products are also a major source for air and water pollution today. To address growing concerns surrounding pollution caused by petroleum and petroleum-based products, many countries have implemented strict regulations on petroleum products, particularly on petroleum-refining operations and the allowable concentrations of specific pollutants in fuels, such as the allowable sulfur and nitrogen content in gasoline fuels. While the exact compositions of natural petroleum or crude oils vary significantly, all crude oils contain some measurable amount of sulfur compounds and most crude oils also contain some measurable amount of nitrogen compounds. In addition, crude oils may also contain oxygen, but the oxygen content of most crude is low. Generally, sulfur concentrations in crude oils are less than about 5 percent by weight (wt %), with most crude oils having sulfur concentrations in the range from about 0.5 to about 1.5 wt %. Nitrogen concentrations of most crude oils are usually less than 0.2 wt %, but can be as high as 1.6 wt %. In the United States, motor gasoline fuel is regulated to have a maximum total sulfur content of less than 10 parts per million weight (ppmw) sulfur, thus the removal of sulfur is a key concern.
Crude oils are refined in oil refineries to produce transportation fuels and petrochemical feedstocks. Typically, fuels for transportation are produced by processing and blending of distilled fractions from the crude oil to meet the particular end use specifications. Because most of the crudes generally available today have high concentrations of sulfur, the distilled fractions typically require desulfurization to yield products, which meet various performance specifications, environmental standards, or both.
The sulfur-containing organic compounds present in crude oils and resulting refined fuels can be a major source of environmental pollution. The sulfur compounds are typically converted to sulfur oxides during the combustion process, which in turn can produce sulfur oxyacids and contribute to particulate emissions, both of which are desired to be reduced.
One method for reducing particulate emissions includes the addition of various oxygenated fuel blending compounds, compounds that contain few or no carbon-to-carbon chemical bonds, such as methanol and dimethyl ether, or both. Most of these compounds, however, suffer in that they can have high vapor pressures, are nearly insoluble in diesel fuel, or have poor ignition quality, as indicated by their cetane numbers, or combinations thereof.
Hydrotreating and hydrogenation are alternate techniques currently used for the removal of sulfur and/or nitrogen from hydrocarbons. Diesel fuels that have been treated by chemical hydrotreating or hydrogenation to reduce their sulfur and aromatics contents can have a reduced fuel lubricity, which in turn can cause excessive wear of fuel pumps, injectors, and other moving parts that come in contact with the fuel under high pressures.
For example, middle distillates (that is, a distillate fraction that nominally boils in the range of about 180-370° C.) can be used as a fuel, or alternatively can be used as a blending component of fuel for use in compression ignition internal combustion engines (that is, diesel engines). The middle distillate fraction typically includes between about 1 and 3 wt % sulfur. Allowable sulfur concentration in middle distillate fractions were reduced to 5-50 ppmw levels from 3000 ppmw level since 1993 in Europe and United States to between a currently allowed amount of about 5-50 ppmw levels from the 3000 ppmw level.
Current conventional techniques for the removal of sulfur and nitrogen compounds typically still require the subsequent disposal of the sulfur and nitrogen compounds that are removed from the hydrocarbons. In order to comply with the increasingly stringent regulations for ultra-low sulfur content fuels, refiners must make fuels having even lower sulfur levels at the refinery gate so that they can meet the specifications after blending.
Low pressure conventional hydrodesulfurization (HDS) processes can be used to remove a major portion of the sulfur from petroleum distillates for the blending of refinery transportation fuels. These units, however, are not efficient to remove sulfur from compounds at mild conditions (that is, up to about 30 bar pressure), when the sulfur atom is sterically hindered as in multi-ring aromatic sulfur compounds. This is particularly true where the sulfur heteroatom is hindered by two alkyl groups (for example, 4,6-dimethyldibenzothiophene). Because of the difficulty in the removal, the hindered dibenzothiophenes predominate at low sulfur levels, such as 50 ppmw to 100 ppmw. Severe operating conditions (for example, high hydrogen partial pressure, high temperature, or high catalyst volume) must be utilized in order to remove the sulfur from these refractory sulfur compounds. Increasing the hydrogen partial pressure can only be achieved by increasing the recycle gas purity, or new grassroots units must be designed, which can be a very a costly option. The use of severe operating conditions typically results in decreased yield, lower catalyst life cycle, and product quality deterioration (for example, color), and therefore are typically sought to be avoided.
Conventional methods for petroleum upgrading, however, suffer from various limitations and drawbacks. For example, hydrogenative methods typically require large amounts of hydrogen gas to be supplied from an external source to attain desired upgrading and conversion. These methods can also suffer from premature or rapid deactivation of catalyst, as is typically the case during hydrotreatment of a heavy feedstock or hydrotreatment under harsh conditions, thus requiring regeneration of the catalyst or addition of new catalyst, which in turn can lead to process unit downtime. Thermal methods frequently suffer from the production of large amounts of coke as a byproduct and a limited ability to remove impurities, such as, sulfur and nitrogen. Additionally, thermal methods require specialized equipment suitable for severe conditions (for example, high temperature and high pressure), and require the input of significant energy, thereby resulting in increased complexity and cost.
Thus, there exists a need to provide a process for the upgrading of hydrocarbon feedstocks, particularly processes for the desulfurization, denitrogenation, or both, of hydrocarbons that use low severity conditions that can also provide means for the recovery and disposal of usable sulfur or nitrogen compounds, or both.